In response to rising demand for information processing services, communications service providers have implemented optical communication systems, which have the capability to provide substantially larger information transmission capacities than traditional electrical communication systems. Information can be transported through optical systems in audio, video, data, or other signal format analogous to electrical systems. Likewise, optical systems can be used in telephone, cable television, LAN, WAN, and MAN systems, as well as other communication systems.
The development of the erbium doped fiber optical amplifier (EDFA) provided a cost effective means to optically amplify attenuated optical signal wavelengths in the 1550 nm range. EDFAs have been widely used in communication systems because their bandwidth coincides with the lowest loss window in optical fibers commonly employed in optical communication around 1550 nm. For wavelengths shorter than about 1525 nm, however, erbium atoms in typical glasses will absorb more than amplify. To broaden the gain spectra of EDFAs, various dopants have been added. For example, codoping of the silica core with aluminum or phosphorus can broaden the emission spectrum. Nevertheless, the absorption wavelength for various glasses is still around 1530 nm.
Raman fiber amplifiers offer an alternative to EDFAs.
Certain optical fibers can be used as fiber amplifiers or fiber lasers.
Fiber amplifiers are typically used to amplify an input signal. Often, the input signal and a pump signal are combined and passed through the fiber amplifier to amplify the signal at an input wavelength. The amplified signal at the input wavelength can then be isolated from the signal at undesired wavelengths.
Raman fiber lasers can be used, for example, as energy sources. In general, Raman fiber lasers include a pump source coupled to a fiber, such as an optical fiber, having a gain medium with a Raman active material. Energy emitted from the pump source at a certain wavelength λp, commonly referred to as the pump energy, is coupled into the fiber. As the pump, energy interacts with the Raman active material in the gain medium of the fiber, one or more Raman Stokes transitions can occur within the fiber, resulting in the formation of energy within the fiber at wavelengths corresponding to the Raman Stokes shifts that occur (e.g., λ1, λ2, λ3, λ4, etc.).
Generally, the Raman active material in the gain medium of a Raman fiber laser may have a broad Raman gain spectrum. Usually, conversion efficiency varies for different frequencies within the Raman gain spectrum and many Raman active materials exhibit a peak in their gain spectrum, corresponding to the frequency with highest conversion efficiency. Additionally, the gain spectrum for different Raman active materials may be substantially different, partially overlapping, or of different conversion efficiency.
Typically, a Raman fiber Laser is designed so that the energy formed at one or more Raman Stokes shifts is substantially confined within the fiber. This can enhance the formation of energy within the fiber at one or more higher order Raman Stokes shifts. Often, the fiber is also designed so that at least a portion of the energy at wavelengths corresponding to predetermined, higher order Raman Stokes shifts (e.g., λsx where x is equal to or greater than one) is allowed to exit the fiber.
Raman fiber amplifiers can be adapted to amplify a broad range of wavelengths.